Natural gas and methane, a major constituent of natural gas, are difficult to economically transport and are not easily converted into liquid fuels, such as methanol, formaldehyde and olefins, that are more readily contained and transported. To facilitate transport, methane is typically converted to synthesis gas (syngas) which is an intermediate in the conversion of methane to liquid fuels. Syngas is a mixture of hydrogen and carbon monoxide with H.sub.2 /CO molar ratio from about 0.6 to about 6.
One method to convert methane to syngas is steam reforming. The methane is reacted with steam and endothermically converted to a mixture of hydrogen and carbon monoxide. The heat sustaining this endothermic reaction is provided by external combustion of fuel. The steam reforming reaction is of the form: EQU CH.sub.4 +H.sub.2 O.fwdarw.3H.sub.2 +CO. (1)
In a partial oxidation reaction, methane is reacted with oxygen and converted to syngas in an exothermic reaction. The partial oxidation reaction is of the form: ##STR1##
Both the steam reforming reaction and the partial oxidation reaction are expensive to maintain. In steam reforming, a significant quantity of fuel is required to provide the heat to sustain the endothermic reaction. In the partial oxidation reaction, significant energy and capital must be expended to provide the oxygen required to drive the reaction.
U.S. Pat. No. 5,306,411 to Mazanec et al., which is incorporated by reference in its entirety herein, discloses the production of syngas by partial oxidation and steam reforming where the oxygen is obtained by transport through an oxygen selective ion transport membrane element and both reactions take place on the anode or permeate side of the membrane. This membrane element conducts oxygen ions with infinite selectivity and is disposed between an oxygen-containing feed stream, typically air, and an oxygen consuming, typically methane-containing, product or purge stream.
"Oxygen selectivity" is intended to convey that the oxygen ions are preferentially transported across the membrane over other elements, and ions thereof. The membrane element is made from an inorganic oxide, typified by calcium- or yttrium-stabilized zirconia or analogous oxides having a fluorite or perovskite structure.
At elevated temperatures, generally in excess of 400.degree. C., the membrane elements contain mobile oxygen ion vacancies that provide conduction sites for the selective transport of oxygen ions through the membrane elements. The transport through the membrane elements is driven by the ratio of partial pressure of oxygen (P.sub.O2) across the membrane: O.sup.-- ions flow from the side with high P.sub.O2 to the side with low P.sub.O2.
Ionization of O.sub.2 to O.sup.-- takes place on the cathode side of the membrane element and the ions are then transported across the membrane element. The O.sup.-- ions then combine to form oxygen molecules or react with fuel while releasing e.sup.- electrons. For membrane elements that exhibit only ionic conductivity, external electrodes are placed on the surfaces of the membrane element and the electron current is returned by an external circuit. If the membrane has ionic as well as electron conductivity electrons are transported to the cathode side internally, thus completing a circuit and obviating the need for external electrodes.
The Mazanec et al. '411 patent discloses contacting an oxygen-containing gas with the cathode side of an oxygen selective transport membrane element. A stream of process gases, such as methane and steam, flows along the anode side of the membrane element. Transported oxygen reacts exothermically with the methane in a partial oxidation reaction forming carbon monoxide and hydrogen. At the same time the heat released by the partial oxidation reaction enables methane and steam to engage in an endothermic reaction to produce additional hydrogen and carbon monoxide. Typically a reforming catalyst is provided to promote this reaction. The syngas can then be converted to methanol or to other liquid fuels by the Fischer-Tropsch process or other chemicals in subsequent processes.
While the Mazanec et al. '411 patent discloses that a portion of the heat generated by the exothermic partial oxidation reaction may be utilized to maintain the temperature of the ion transport membrane element, no provisions are made for the removal of excess heat from the reactor. Further, while the partial oxidation and steam reforming reactions are best conducted at high pressure, there is no disclosure in Mazanec et al. of a reactor design or sealing configuration to support high pressures.
Commonly owned U.S. patent application Ser. No. 08/848,204 now U.S. Pat. No. 5,820,655 entitled "Solid Electrolyte Ion Conductor Reactor Design" by Gottzmann et al. that was filed on Apr. 29, 1997 and is incorporated by reference in its entirety herein, discloses using the heat generated by an exothermic partial oxidation reaction to heat an oxygen-containing feed gas prior to delivery of that feed gas to the cathode side of an oxygen selective oxygen transport membrane element. The Ser. No. 08/848,204 application also discloses the use of a thermally conductive shroud tube surrounding the membrane elements to enhance the conduction of heat while maintaining isolation of gases. Reactive purge arrangements are disclosed in "Reactive Purge for Solid Electrolyte Membrane Gas Separation", U.S. Ser. No. 08/567,699, filed Dec. 5, 1995, E.P. Publ. No. 778,069, and also incorporated herein by reference. Both applications are commonly owned with the present application.
U.S. Pat. Nos. 5,565,009 and 5,567,398 to Ruhl et. al., that are incorporated by reference in their entirety herein, disclose manufacturing syngas by steam reforming of methane in a catalyst bed located on the shell side of a tube and shell reactor. The heat for sustaining the reforming reaction is provided by combustion of fuel within tubes where the fuel and oxygen supply (air) are separately heated and only combined after they reach their autoignition temperature. The flow paths of the reactor disclosed by Ruhl et al. are arranged such a way that the combustion products as well as the endothermic reaction products are cooled before exiting the furnace. The disclosed design allows for the use of lower temperature seals where the combustion tubes are joined to tube sheets.
There remains, however, a need for a reactor for the production of syngas and unsaturated hydrocarbons that utilizes an oxygen-selective ion transport membrane element, is capable of operating at pressures above 150 psig and temperatures in the range of 800.degree. C. to 1100.degree. C., and has provisions that compensate for dimensional changes in the membrane elements due to thermal heating and due to the uptake and release of oxygen by the membrane elements during operational and transitional periods. The reactor should, in addition, maintain the membrane elements within prescribed temperature limits by careful balance of the heats of reaction and other heat sinks or sources as well as effective transfer of heat from exothermic reactions to endothermic reactions and other heat sinks. It should also increase safety by minimizing the risk of a high-pressure leak of a flammable process or product gas into oxygen containing streams.